The present invention relates to underwater buoyancy apparatus of the kind having an expansible gas pocket for providing the required buoyancy, which apparatus is referred to hereinafter as "expansible buoyancy apparatus". The invention has particular, but not exclusive, application to diver's lifejackets, dry suits, and underwater lifting bags.
The buoyancy of expansible buoyancy apparatus is provided initially by admitting gas, usually air, to the gas pocket of the apparatus. In a diver's lifejacket, the gas pocket usually is constituted by a closed flexible container which is inflated by the gas. The flexible nature of the container permits of expansion of the gas within the container. Lifting bags usually comprise an open-ended flexible bag into which the gas is admitted. In some cases, the whole bag can constitute the gas pocket in similar manner to a lifejacket. However, in other vases the gas pocket can be consituted by the volume occupied by a gas defined between the bag and water contained in the bag. In the latter case, expansion of the gas pocket to displace water through the open end of the bag.
It is difficult to control the ascent of expansible buoyancy apparatus because the progressive decrease in water pressure upon ascent of the apparatus causes progressive expansion of the gas chamber and hence a progressive increase in buoyancy, which in turn causes accelerated ascent of the diver or article to which the buoyancy apparatus is attached. This is a particular problem in the case of lifting bags where acceleration of the bag and the article being lifted constitutes a significant risk to those operating the lifting bags.
A device intended to vent excess buoyancy from a lifting bag is described as part of a hover control system in U.S. Pat. No. 3,659,299 (Davidson et al.).
Davidson provides a venting valve mounted on the top of a lifting bag and intended to be opened by dynamic pressure of water caused by upward movement of the bag in water deflecting a diaphragm mechanically linked to the valve member of the venting valve. The venting valve is biassed shut by a spring of adjustable tension. In practice, the magnitude of the dynamic pressures needed to open the venting valve taught by Davidson means that the valve will not open to produce venting until the upward velocity of the bag is already excessive and the venting produced may in those circumstances not be fast enough to control the ascent. Davidson's apparatus will not in fact produce the hovering action he describes.
U.S. Pat. No. 2,887,976 describes the use of a diaphragm exposed on one side to ambient water pressures as a means of sensing static pressure and operating a venting system to maintain pressure differential between a ballast tank and ambient within a permitted range. The static pressure sensor can be used to act directly on the venting valve because the forces available are greater than when sensing the dynamic pressure produced by ascent of a lift bag or lifejacket. Of course, the role played by the static pressure sensor in the apparatus of Hanna could not be played by a dynamic pressure sensor and Hanna's apparatus could not function if a dynamic pressure sensor were used.